A typical transmitter optical subassembly (TOSA) includes an opto-electronic TO package that contains a laser and a lens, and a receptacle that contains a fiber stub. The TO package and the receptacle are coupled together such that the output of the laser is focused through the lens on the core of the fiber stub.
In TO packages for un-cooled lasers, a package configuration called vertical TO package is frequently used. In a vertical TO package, a laser chip on a substrate, e.g., an edge emitting laser diode, is mounted substantially flat on a TO header rather than being mounted at substantially 90 degree angle with respect to the TO header on a flange. This results in a reduction to the length of thermal path between the laser chip and a contact surface (for heat sink) in the TO package. Therefore, a laser chip under the same condition can typically operate at a temperature of 5 to 7 degrees less than the case where the laser chip is mounted on a flange.
Since the laser output from the TO package should point vertically away from the TO header, in vertical TO packages, the laser propagating direction is folded from the horizontal (i.e., generally parallel to the TO header) to vertical (i.e., generally perpendicular to the TO header) using a reflective mirror. In typical vertical TO packages, the mirror cut angle is selected to be 45 degrees.
The fiber stub is typically cleaved at the receiving end so that a crossing angle exists between the central line of the fiber stub and that of the laser output. This way, the interference between the reflected laser output and the laser output may be reduced, thereby resulting in reduction to optical return loss. In order to ensure that the laser output is incident on the cleaved edge of the fiber stub at a proper angle, the center of the focus lens is typically moved off the centerline of the optical system. However, when the center of the focus lens is moved off the centerline of the optical system, a chief ray of the laser output does not overlap an optical axis of the lens. In such systems, a large amount of off-axis aberration (coma, astigmatism, etc.) may result, and thereby may reduce optical coupling efficiency.